The invention relates to a propulsion control system for motor vehicles or the type shown in U.S. Pat. No. 4,484,280.
It concerns a wheel slip control (ASR) for the driven wheels of a vehicle having a driven axle, a slip comparator being assigned to each of these wheels. If one driven wheel exceeds a given slip value, this wheel is braked. If both driven wheels exceed this slip value, output torque of the vehicle engine is reduced. In one variation, the torque is reduced if only one drive wheel is spinning.
Because of the fixed slip threshold value, a wheel slip control occurs earlier than required. As a result of the differing rotational speeds of the vehicle wheels during turning, these slip values are reached much earlier than in the case of straight-ahead driving. Also, the road condition and the relationship between the road and the vehicle are not taken into account.
It is therefore the objective of the invention to improve the known system to the extent that the slip threshold values are adapted better to momentary conditions and adhesion coefficient or the moment that can be transferred to the road, and to the extent that the control is refined.
The advantages of the invention are that the measured longitudinal acceleration in connection with the working propulsion control exhibits a rough relationship to the adhesion coefficient and can therefore be used as an auxiliary signal. Particularly in the case of all-wheel driven vehicles having an ASR, this interrelationship is rather exact and is essentially erroneous due to the diagonal position in longitudinal direction of the vehicle coordinates during turning. During turning, in addition to the diagonal position of the vehicle coordinates, the loading of the vehicle also enters into the precision of the measurements. In both cases, however, the measuring of the longitudinal acceleration alone is sufficient, if the steering angle is also used in logic processes to select the threshold value of the wheel slip control. In this case, particularly the possibility is considered of changing the switching thresholds of control signals, such as positive and negative wheel accelerations and the like, to the extent that such control signals are used in the propulsion control system.
Another advantage of the use of the longitudinal acceleration signal is the fact that it can be used for the changing of the rate of adjusting the brake pressure for the ASR-controlled wheels and the power control element of the vehicle engine. In the case of low longitudinal acceleration, the falling of the brake pressure and the rerising of the engine torque, after an ASR-intervention, take place slowly, so that the controlled wheel with the low torque excess is guided in the direction of the stable area. In the case of a high longitudinal acceleration, and thus higher adhesion coefficients, the rate of adjusting may be considerably higher because in this case, also in the case of powerful engines, the excess amount is not too high. This control has special advantages in the case of inhomogeneous roads, particularly when so-called .mu.-jumps occur, because then the ASR-system becomes operative for a short time. The longitudinal acceleration, with corresponding additional expenditures, may also be derived from a driving speed sensor. Sensors are known that work according to the Doppler radar principle or optical correlation. Also, with a compromise in regard to precision, the longitudinal acceleration can be derived from the change of the averaged wheel speeds with respect to time.
The circuitry according to the present invention to produce this object include two additional slip comparators, one corresponding to a lower and one to a higher indicated threshold value for each driven wheel and an additional longitudinal acceleration comparator adjusted to an indicated higher threshold value. A logic circuit is provided for alternatively actuating one of the three slip comparators when certain combination is present of:
(a) the output signal of the speed comparator,
(b) one of the longitudinal acceleration comparators, and
(c) steering angle comparator as well as an input signal of the actuating device for the wheel brakes and of the actuating device for the control element affecting the engine torque.
The lower threshold slip comparator is actuated when signals are simultaneously present from the speed comparator, from the longitudinal acceleration with the higher threshold and not from the longitudinal acceleration with the lower threshold, from the steering angle comparator and from at least one of the actuating devices. The higher threshold slip comparator is actuated when the output signals are simultaneously present form the steering angle comparator and from the longitudinal acceleration with the higher threshold value. The slip comparator with the medium threshold value is actuated when actuating signals are not present for the other two slip comparators. The speed of change of the brake pressure and of the adjusting speed of the power control element for the vehicle engine is adjusted if output signals are simultaneously present from the speed comparator, from the longitudinal acceleration comparator having the higher threshold value and at least one of the actuating devices.